It was not immediately known if he quit or was fired. Olbermann did not address the question, and MSNBC said only that they and Olbermann had ended their contract. He signed a four-year contract two years ago.
"MSNBC thanks Keith for his integral role in MSNBC's success and we wish him well in his future endeavors," the network said in a statement.
A spokesman said Phil Griffin, MSNBC's president, would not comment on Olbermann's exit. Spokesman Jeremy Gaines would say only that the acquisition of NBC Universal by Comcast, which received regulatory approval this week, had nothing to do with the decision.
Olbermann was suspended without pay from the network for two days in November for donating to three Democratic candidates, which violated NBC News' policy on political donations. Olbermann complained that he was being punished for mistakenly violating an inconsistently applied rule that he had known nothing about.
The host apologized to fans — but not to the network.
MSNBC essentially molded the network in Olbermann's image. His program is MSNBC's top-rated, gaining in viewers after his evolution from a humorous look at the day's headlines into a combatively political show in the latter days of the Bush administration. MSNBC decided that point-of-view programming was the way to go, and hired Rachel Maddow and Lawrence O'Donnell — both occasional subs for Olbermann — to fill out its prime-time lineup.
His exit was so sudden that MSNBC didn't have time to change its ads; a "Lean Forward" promotion for the network featuring Olbermann aired within a half-hour of his final goodbye.
Olbermann, before leaving the show with a final signature toss of his script toward the camera, thanked his audience for sticking with him. As was often his habit on Friday nights, he read a James Thurber short story, this one titled "Scottie Who Knew Too Much" and published in 1940.
The story's final line: "It is better to ask some of the questions than to know all of the answers."
"There were many occasions, particularly in the last 2½ years, where all that surrounded the show — but never the show itself — was just too much for me," said Olbermann, whose father died recently. "But your support and loyalty and, if I may use the word, insistence, ultimately require that I keep going. My gratitude to you is boundless."
He said he was grateful to the network that he was given time to sign off, noting that when he left ESPN in the 1990s, he was given 30 seconds — cut in half at the last minute to get in tennis results.
But Olbermann was known for a mercurial personality behind the scenes and he was almost fired last year for the political donations. He quit a prime-time show on MSNBC in the late 1990s, complaining that management was making him report too much on President Bill Clinton's impeachment scandal.
He was particularly critical of Fox News Channel and his direct competitor, Bill O'Reilly, frequently naming him his "Worst Person in the World" in a segment popular with his fans. Bosses at NBC had discussed trying to keep the tone of the vitriol down.
Olbermann came in to work after the shooting of Arizona Rep. Gabrielle Giffords, airing a special comment in which he promised to tone down inflammatory rhetoric and challenged others to do the same.
It's unclear what Olbermann's plans are now.
He could give a boost to struggling CNN's prime-time lineup, but Olbermann would mean CNN would make an abrupt shift in its nonpartisan policy. It was not immediately known how quickly Olbermann could switch to another job if he wanted to; such contract buyouts typically include noncompete clauses.
"Keith is an innovator and extremely talented broadcaster who showed there was a market for progressive views on cable news," Brock said. "I'm sure we'll be hearing more of him soon, and I eagerly await hearing of his next move."
The mood was different at Newsbusters, a website operated by the conservative Media Research Center: "You guys at Newsbusters should really break out the champagne and party," wrote one reader about Olbermann.
Watch Olbermann sign off for the last time here and comment below: